1. Field of the Invention
The present invention relates to a light-emitting diode driving apparatus and a light-emitting diode driving operation controlling method, and in particular to a light-emitting diode driving apparatus and a light-emitting diode driving operation controlling method using AC power supply.
2. Description of the Related Art
In recent years, significant attention is given to light-emitting diodes (hereinafter, occasionally referred to as “LEDs”) as lighting sources. The reason is that LEDs can be driven at low power consumption as compared with filament lamps or fluorescent lamps. LEDs are small, and have shock resistance. In addition, LEDs are less prone to burn out. Thus, LEDs have these advantages.
In the case of lighting sources, it is desirable that AC power such as commercial power for home use is used as power supply for lighting sources. LEDs are devices driven by DC power. LEDs emit light only when applied with a current in the forward direction. Also, in the case of LEDs that are currently typically used for lighting use, the LEDs operate on DC power at a forward directional voltage Vf of about 3.5 V. LEDs do not emit light if a voltage applied to the LEDs does not reach Vf. Conversely, a voltage applied to the LEDs exceeds Vf, an excessive amount of current will flow through the LEDs. Accordingly, it can be said that DC power is suitable for driving LEDs.
To satisfy the contradictory conditions, various types of LED driving circuits have been proposed that use AC power. For example, in a driving circuit shown in FIG. 8, after an AC voltage of an AC power supply 71 is subjected to full-wave rectification in a bridge circuit 72, and is then smoothed by a smoothing capacitor 73, an LED group 75 is driven by a driving circuit 74 consisting of a constant current circuit, a switching power supply circuit and the like. In this circuit, since the smoothing capacitor 73 is required to have a high voltage resistance and a high capacitance, this circuit necessarily has a large element such as aluminum electrolytic capacitor. Also, generally, the life of electrolytic capacitors will be short, in the case where the ambient temperature is high. A coil used in the switching power supply also will be large and deteriorate under high temperature condition. Since the switching power supply circuit switches very quickly between full-on and full-off states at a large amount of current, noise is likely to be generated. Accordingly, noise control measures are required. For this reason, this driving circuit is required to prepare space for elements of the driving circuit for driving LEDs, which could be essentially suitable for size reduction. In addition, this driving circuit is required to have a temperature shielding structure and noise control measures.
To address these problems, driving methods are devised that drive LEDs by using a constant current circuit or the like without smoothing a voltage waveform rectified by the bridge circuit. FIG. 9 shows a circuit diagram of this type of circuit. In this illustrated drive circuit, after an AC voltage of an AC power supply 81 is subjected to full-wave rectification in a bridge circuit 82 similarly to FIG. 8, an LED group 85 is driven by a constant current circuit 84 consisting of transistors and resistors without smoothing. The constant current circuit 84 consists of a feedback resistor 86, a current detection transistor 87, a current control transistor 88, and a current detection resistor 89. Since this circuit consists of semiconductor elements, this circuit can operate in the same operating temperature range as LEDs, which are also semiconductor device. Accordingly, it can be said that this circuit is suitable for size reduction.
However, in the case where LEDs are driven without smoothing, the voltage of waveform is not fixed but periodically varies as shown in FIG. 10. LEDs are connected to each other in series as shown in FIG. 9. Accordingly, the LEDs do not emit light as long as a voltage applied to the LEDs exceeds the total value of the forward directional voltages Vf of the LEDs. For this reason, in the case of a voltage waveform that varies in accordance with time, the emission time of the LEDs is limited. As a result, the operation efficiency of LED decreases. Here, the operation efficiency of LED refers to a value represented by (effective power consumption of LED)/(power consumption of LED when driven at DC rating).
In particular, in the case of a circuit that includes a current restriction resistor connected to an LED in series to protect the LED, the electric power of the LED also sharply varies in accordance with power supply voltage variation. Considering that, in some cases, a current flowing through the LED may exceed the current rating of the LED, it is necessary to previously adjust a current flowing through the LED to a smaller value. For this reason, in this case, a constant current circuit is typically incorporated to drive the LED. In more detail, in this case, for example, since the effective value of commercial power is 100 V in Japan, the maximum voltage after full-wave rectification is 141V. In the case where an LED(s) is/are connected to this power supply through the constant current circuit and driven by the constant current circuit, when only one LED with Vf=3.5 V is connected to this power supply through the constant current circuit and driven by the constant current circuit, the LED is turned ON in a range where the power supply voltage exceeds 3.5V. Accordingly, the LED operation efficiency will be high. However, as shown by shaded areas in the voltage waveform in FIG. 11, most of electric power will be consumed to generate heat, and as a result is not used for light emission. Accordingly, power supply efficiency will greatly decrease.
Also, it is conceivable that a plurality of LEDs are connected in series to this power supply so that the number of connected LEDs is increased whereby adjusting the total value of the forward directional voltages Vf to a value near 141 V. In this case, if the power supply efficiency is adjusted to about 90%, a Vf total value of about 120 V is required. However, in this configuration, the LEDs are turned ON only when the power supply voltage exceeds 120 V. The LEDs do not emit light in a range in that the power supply voltage does not reach 120 V. Accordingly, the LEDs only emit light in ranges shown by dashed lines in FIG. 11. As a result, the ON duty of this configuration will be only about 35%. For this reason, the LED operation efficiency also will be only about 35%, and the power factor will be only about 77%. As discussed above, if the Vf total value is adjusted small to increase the LED operation efficiency, power will be wasted to generate heat. Conversely, if the Vf total value is adjusted large to improve the power supply efficiency, the LED ON-duty will be small. As a result, the LED operation efficiency decreases. These requirements are contradictory to each other.
A method has been proposed that switches LEDs so that a Vf total value is changed in accordance with a varying voltage value (see Japanese Patent Laid-Open Publication No. 2006-147933). In this method, a number of LEDs connected to each other in series are divided into blocks 61, 62, 63, 64, 65 and 66 as shown in a circuit diagram of FIG. 12. The LED blocks 61 to 66 are selectively connected to the power supply in accordance with the voltage value of input voltage of rectified waveform by a switch controlling portion 67 consisting of a microcomputer so that a Vf total value is changed in a stepped manner. As a result, as shown by a voltage waveform in a timing chart of FIG. 13, since the LEDs can be driven by a plurality of rectangular waves corresponding to the rectified waveform, the LED operation efficiency can be improved as compared with the ON-duty in the case of only single rectangular wave shown in FIG. 11. However, in this method, since the microcomputer is used to select connect the LED block based on the result of a detected voltage value of input waveform, complicated control is available but the circuit configuration becomes expensive. For this reason, this method is not suitable for inexpensive lighting apparatuses.
Also, an apparatus has been proposed that detects a voltage by Zener diodes and resistors without a microcomputer as shown in a circuit diagram of FIG. 14. In this illustrated circuit, LED blocks 91, 92 and 93 are selectively connected to the power supply in accordance with a voltage value of input voltage of rectified waveform based on a voltage value obtained by dividing a power supply voltage by Zener diodes 94 and resistors 95 so that a Vf total value is changed in a stepped manner. As a result, LEDs can be driven by a plurality of rectangular waves corresponding to the rectified waveform as shown by voltage waveforms in FIG. 15. This apparatus can be configured inexpensive as compared with the circuit configuration shown in FIG. 12.
However, in the aforementioned both proposals, since the LED blocks are selectively driven in accordance with a rectified input voltage, the threshold voltage value is required to accurately match with a total Vf value of each LED block (at a specified current). Generally, LED devices have property deviation. LED devices have Vf values and temperature characteristics different from each other. For this reason, it is very difficult to accurately adjust a total Vf value of each LED block in fact. In particular, since a plurality of LED devices are necessarily connected to each other in series in each LED block as shown in FIG. 13, deviated Vf values of the devices are summed. As a result, a total deviated Vf value of the entire LED block will be further increased. Although it is conceivable that only previously sorted LED device are used to suppress the deviation, this may increase costs of LED devices and deteriorate yields of LED devices. In particular, a number of LED devices are used in a lighting apparatus. Accordingly, cost reduction of LEDs is strongly required to spread the use of LED lighting apparatus. For this reason, such LED device sorting is not actually available.
In the case where a Vf total value of an LED block deviates from the desired value, if the Vf total value is higher than the threshold voltage value, even when LEDs in the LED block are selectively connected to the power voltage, the LEDs cannot emit light. This causes noise generation and power factor reduction. Conversely, in a Vf total value of LEDs is lower than the threshold voltage value, a corresponding excess amount of power will be wasted in the constant current circuit. For this reason, because of LED device deviation, it is difficult to provide desired LED device operation. As a result, selective light emission delay may occur and the efficiency may decrease. Accordingly, in fact, it is difficult to realize selective light emission in terms of LED light emission quality and reliability.
In the aforementioned method, although the LEDs can be driven by a plurality of rectangular waves by selectively connecting the LED blocks to the power supply, power is still wasted as shown by diagonally shaded areas in FIG. 15. For this reason, the efficiency of the aforementioned method is still poor.
In particular, although LEDs can essentially emit light at the highest intensity in a part where the highest voltage is applied in the area. However, such a range is not effectively used.